Aperture Placement & Area

“Aperture” refers to any daylight source, including windows, skylights, openings, and any other transparent or translucent surfaces.

Aperture placement and area is important because strategic use of windows and skylights can help you achieve thermal and visual comfort passively, saving both energy and money.

Glazing placement and area can be measured in 3D models or 2D drawings and is important for both visual and thermal comfort.

Aperture Area

Side Light

Bigger apertures are not necessarily better. They can cause too much heat loss or heat gain, or too much brightness and glare. Choosing just the right sizes for apertures (“Right-sizing”) is key. One way of measuring apertures is the Window-to-Wall Ratio (WWR):

Here, the “net glazing area” refers to only the transparent part of the window, not mullions or framing (usually net glazing area is around 80% of gross window area), and the “gross wall area” uses the full floor-to-floor exterior height of the wall. A common rule of thumb states that the window to wall ratio should be 40% or lower for adequate insulation in cold climates, though more advanced windows with higher R values (lower U values) allow higher ratios. In warm climates, higher ratios can be acceptable even without well-insulated windows, as long as the windows are well shaded from the sun’s heat.

Different window-to-wall ratios and the resulting illumination

Another metric to pay attention to for proper glazing from side windows is the Window-to-Floor Ratio (WFR). A rule of thumb for side-lighting thresholds is that the Window-to-Floor ratio, multiplied by the visible light transmittance (VLT, or Tvis) of the windows, should be:

0.15 < VLT • WFR < 0.18

Top Light

Toplighting apertures are much brighter than sidelighting apertures, so less area is required. Similar to the Window-to-Wall-Ratio, there is a Skylight-to-Roof Ratio (SRR) that is the net glazing area divided by the gross roof area. A rule of thumb1 is that the SRR should be between 3% and 6%. Tubular skylights require a much lower SRR than traditional skylights, approximately 1-2%.

To size a rectangular skylight, you can use this simple formula:

Area of one skylight = (Floor to Ceiling Height x 1.5)2 • target SRR

Choose a size that is appropriate. Start with 5% SRR and modify depending on climate and building use. For example with a 12′ ceiling and 5% skylight to roof ratio the right size skylight would be approximately: (12 x 1.5)^2 x 5% = 16.2sf. Therefore the project should use 4’x4′ or 8’x2′ skylights for good light distribution.

Aperture area is not the entire story, however. The right size for apertures depends on their placement in the building, the building’s orientation, and the glazing properties.

Aperture Placement For Daylighting

The locations of apertures matter a great deal. As mentioned in Building Orientation, windows and other openings facing the path of the sun receive much more direct sunlight than those facing away. However, more daylighting is not necessarily better. Bringing in too much light can cause glare and overheating. See Measuring Light for Lighting and Daylighting Design.

Evenly-distributed light is critical to good daylighting, so apertures that are evenly distributed are useful. Continuous-strip apertures are even better, and apertures on multiple sides are often best. Otherwise rooms can have “hotspots”, both in terms of temperature and brightness. Often this is accomplished with horizontal bands of windows that are placed high in a space (to avoid glare and reflect light off the ceiling), or with evenly spaced vertically oriented windows that reach the full height of the room.

Evenly-spaced windows on two walls provide well-distributed light

(See more on Anderson Anderson ZEBRA project)

Side Light

Light coming from side apertures like windows can only penetrate so far into a building. This is the reason why shallow floorplans are usually recommended for daylighting multi-story buildings. A simple rule of thumb for most latitudes is that daylight penetrates into a room roughly 2.5 times the height of the top of the window. See the Ecotect Community Wiki for more examples of different geometries.

Side lighting only reaches so far into a room

Windows facing away from the sun’s path rather than towards the equator provide the most even illumination, though not the brightest. East and west facing windows can provide very bright light in the morning or evening but insufficient light at other times of day, and are very prone to glare. Windows facing towards the sun’s path get the brightest light; they can also have glare, but the glare is much easier to control than on East or West walls.

Windows on different faces have different amounts of useful daylight (depicted with straight arrows) and unhelpful glare (depicted with jagged arrows).

In middle latitudes and those closer to the equator, skylights can provide the brightest and most consistent illumination, but in latitudes closer to the poles they are less bright and much less seasonally consistent.

Top Light

Higher apertures are more effective at bringing light deep into the building. This often means glazing in roofs.

Skylights are not the only kind of aperture to bring light in through roofs. Other “toplighting” strategies include clerestories, monitors, and sawtooths or other scoop-shapes.These each have their own advantages and disadvantages in construction cost and how they bring the sun into the space at different times of day and year.

Different kinds of toplighting

Top lights are usually much brighter than side lights per unit area, for the same glazing properties. A rough approximation for moderate latitudes is that a vertical monitor brings in twice as much light as a view window, an angled monitor brings in three or more times as much depending on the angle and a horizontal skylight brings in five times as much light as a view window.

Baffles are often used in toplighting to help direct the light usefully into the room, and splaying the edges of openings can help the light spread more broadly though the space.

Two kinds of splayed openings

(Image from Sun, Wind, and Light by G.Z. Brown and Mark DeKay)

Daylight Apertures vs. View Windows

Good daylighting design considers daylighting apertures separately from view windows, even when the same window is used for both. Daylighting windows have different optimal sizes, placement, and glazing properties from view windows. Windows used for both functions usually compromise performance.

Daylighting apertures are best located as high as possible on the walls or ceiling, for deeper penetration of light into the space. View windows, however, must be at eye level for occupants, both sitting and standing. View windows are usually preferred to be large, while daylighting windows can be smaller.

While daylighting windows ideally diffuse the light, view windows must have clear views. This makes the avoidance of glare a top concern for view windows. Often shades and/or light shelves are placed between daylighting windows and view windows, to shade the view window while diffusing and redirecting light from the daylighting window. Also, view windows may be more heavily tinted than daylighting windows to avoid glare.

Daylighting window vs. view window

Aperture Placement For Heating & Cooling

Windows and other apertures bring in heat from sunshine, but can also lose heat by radiative cooling and by conducting heat better than most wall or roof constructions. Apertures and shading must be intelligently placed to take advantage of the sun’s heat in cold locations and seasons, while not overheating in hot seasons.

Placement matters for heating and cooling in much the same way it matters for daylighting. Openings facing towards the sun’s path or on top of the building can pull in large amounts of heat – usually more than east or west apertures.

As with daylighting, glazing with the right properties must be chosen for the right orientations. For example, a window that lets in plenty of useful heat when located in one side of the building might allow terrible heat loss if located in another side.

However, there are some differences to consider when designing apertures for thermal comfort. Apertures facing away from the sun’s path usually lose heat to the outside. In many climates, east windows can be desirable to warm spaces early in the day after a cold night. In warmer climates, west-facing windows can be especially prone to overheating.

Passive solar heating usually combines solar heat gain with thermal mass inside the building, to capture and store the sun’s heat for slow release through the night.